Thursday, March 17, 2016

Pre-coating Plywood

Recently, I was contacted by Phoenix III builder, Jonathan McNally regarding some persistent cracking he has noticed in his boat where the garboard strake (i.e. the plank closest to the keel) overlaps, and is glued to, the keelson.

Photograph of the keelson of Jonathan McNally's Phoenix III , where you can just see a feint crack in the outer edge of the epoxy fillet on the inside surface of the garboard strake.
According to Jonathan's report, he has re-epoxied this section several times, but the slight cracking keeps coming back. He and I have discussed the matter by 'Trans-Pacific' email, and have diagnosed the problem, tracing it back to (we believe) the high-quality, but unusually flexible plywood used - that is another story.

However, Jonathan's story brings up several matters which I'd like to discuss regarding the use of epoxy as an adhesive, and as a surface sealant i.e. epoxy encapsulation. For those who have missed it, here is a link to a recent blog post I put up about the hull structure of Phoenix III after Jonathan first reported his problem.

Two of the possibilities I had considered in regard to Jonathan's cracking problem were: -

  • perhaps the planks had been pre-coated with epoxy, and when glued into position on the boat, the cured epoxy coating may not have been adequately sanded; and/or
  • the pre-thickened epoxy glue may not have been laid onto a freshly primed gluing surface.
Neither of those possibilities were to blame as it turned out, but they do bring up issues about which people need to be aware.

Epoxy is my favourite marine adhesive by far, although I do make use of a number of other glues for specific jobs. But epoxy is the most versatile adhesive I use, and the vast majority of my gluing is done using epoxy and suitable additives. Epoxy is gap-filling in a truly structural sense, and that is the key to its versatility.

 When using epoxy as an adhesive and/or as a filleting agent it is really important that all of the gluing surfaces be primed with an application of un-thickened epoxy resin/hardener. This relatively low-viscosity application will penetrate the surface of the timber and form an excellent foundation to which the thickened adhesive mixture will bond chemically. For this to occur, the priming application should be applied no more than a few hours before the adhesive, so that it will still be chemically active when the adhesive mixture is applied.

Here you can see how I have primed (or wet-out) two adjoining surfaces prior to laying down thickened epoxy which will be formed into a fillet.

In this photo, the joints on the left have had the epoxy formed into a fillet over the primed surfaces of the joint, and glass tape has been placed over the fillet, and wet-out with another application of un-thickened epoxy. The lady on the right is brushing epoxy through glass tape which has been laid into the still wet epoxy priming coat and the wet thickened epoxy fillet.

On face grain, epoxy does not penetrate a long distance - I've heard various distances mentioned, from fractions of a millimetre to as much as a millimetre in the case of some very porous timbers - but on a molecular scale it is a very substantial distance, and the epoxy adhesive will adhere tenaciously. In end-grain, epoxy penetrates a much longer distance indeed.

Now this brings me onto the subject of pre-coating plywood - or any wood for that matter - and what I see as being some stumbling blocks. Pre-coating sheets of plywood laid flat on a bench is certainly convenient, and efficient from the coating application perspective. But the problem is that when the components are cut from those pre-coated sheets, all surfaces which are going to be glued MUST be very well abraded so that the epoxy adhesive (and its priming coat) have a 'key' or 'tooth' to which a mechanical bond can be established. This represents an extra step in the building process, and detracts from gains made through the pre-coating. Also, the mechanical bond between the fresh epoxy and the previously applied pre-coat represents a 'secondary' bond - good if well executed, but not as good as a chemical bond.

I also have concerns about the cured epoxy on pre-coated sheets being subjected to tension and compression when components are bent into position. In my mind's eye, I see micro cracks forming on the tension side of the material, and crushing occurring on the compression side. Unfortunately, I do not have the engineering or chemical qualifications to claim that I know what I'm talking about!

Under some circumstances there may be a place for pre-coating - an example would be the under-surface of a cabin-top or a deck, where subsequent sanding would be very difficult. In that situation, the under-surface could be pre-coated, and then sanded to the point where it is ready to accept adhesive where it sits on deck-beams etc, and it would also be ready to accept paint.

Other than in the cases mentioned in the preceding paragraph, I much prefer to build the boat structurally, and then apply any epoxy coatings. I have fairly strong opinions about where epoxy coating is of value, but that can be the subject of another post.